As is known in the art, many applications, ranging from industrial plasma generation to wireless power transfer, require inverters (or power amplifiers) that can deliver power at high frequency (HF), e.g. 3-30 MHz or Very High Frequency (30-300 MHz) or above to a load circuit (or more simply, a “load”). Such applications often utilize signals having frequencies which fall with the industrial, scientific and medical (ISM) band of frequencies (e.g., 6.78 MHz, 13.56 MHz, 27.12 MHz). The loads coupled to an output of the inverter to receive HF power therefrom may often exhibit load impedances that vary over a wide range. Such impedance variations include variations in both inductive and capacitive impedance characteristics as well as resistive variations.
As is also known, addressing such applications with circuits and systems which operate at a relatively high efficiency is challenging owing to the constraints imposed by the combination of HF operation and varying impedances of loads. Inverter designs at HF generally utilize fundamental-frequency inductive loading of the inverter transistor(s) to achieve zero-voltage switching (ZVS) transitions necessary to achieve high efficiency operation. Also for efficiency reasons, it is desirable to provide only a minimum amount of inductive loading necessary to support ZVS along with the current needed to support the load. Providing HF power to a load having a highly-variable load impedance (particularly into a load impedance having variations in inductive and capacitive impedance characteristics in addition to resistive variations) makes it difficult to maintain desired inductive transistor loading without requiring a large inductive circulating current, which itself can induce substantial loss. Thus, variations in load impedances (i.e. loading variations) can directly constrain an achievable operating range and efficiency of an inverter system. Furthermore, these constraints become increasingly severe with increasing frequency and power ratings.
One approach to address load impedance variations in some applications is to augment an inverter designed for a single load impedance (e.g., 50 Ohms) with a tunable matching network (TMN) that dynamically matches the variable load impedance to the fixed value desired for the inverter. Such TMNs realize the adaptive tuning using variable passive components, such as motor-driven mechanically-variable capacitors, switched capacitor banks, or high-power varactors. This approach is very effective since it allows the inverter to operate at its designed operating point for all loads within the tuning range of the TMN. The TMNs themselves, however, are generally expensive, bulky, slow and inefficient. Thus, efficient generation and delivery of power into variable load impedances is difficult, resulting in HF inverter (or power amplifier) systems that are bulky, expensive and inefficient.